1. Field of the Invention
This invention pertains to a source of RF power utilizing semiconductor devices and more particularly to a source of microwave/millimeter wave signals utilizing an optically triggered Gunn effect device.
2. Description of the Prior Art
The use of gallium arsenide (GaAs) and indium phosphide (InP) semiconductors as a source of RF power is well known, having been described, for example, in an article by J. B. Gunn entitled, "Microwave Oscillations of Current in III-V Semiconductors", which appeared in Solid State Communications, Vol. 1, pp. 88-91 (1963). In that publication, the Gunn effect is disclosed. Gunn oscillations are produced when n-type GaAs, for example, is biased into negative mobility region. The negative mobility gives rise to carrier bunching which are then swept through the drift region giving rise to RF oscillations. The oscillation frequency is approximately equal to the reciprocal of the transit time across the drift region, typically 1 GHz for a sample with a gap length of the order of 0.1 millimeter. These oscillations, moreover, appear as an RF signal superimposed on a video pulse and appear when the fields across the semiconductor are in the range of 2-4 kV/cm.
Optical control of Gunn effect devices is also known. However, in such devices, an optical signal is used either to: (1) permit operation at lower voltages, (2) provide frequency stabilization, or (3) provide frequency flexibility, or otherwise improve the range or efficiency of the oscillator. Such optically controlled devices provide no increase in output power. The devices, moreover, are fabricated from relatively low resistivity material which is in the range of 2 to 10 ohms-cm. Some of these devices are initially biased below the threshold field for oscillation (approximately 3 kV/cm) and are then selectively illuminated with light of a wavelength which is suitable for carrier generation in the specific materials. Increased conductivity in the illuminated region causes the field to collapse in that region and correspondingly to increase to a value above the oscillation threshold in the unilluminated region. The RF oscillations then begin in the unilluminated region and propagate through the device. On the other hand, some of the known devices are biased at or slightly above the threshold electrical field for oscillation and allowed to oscillate prior to optical stimulation. Uniform or selective illumination is then employed to vary oscillation frequency, stabilize oscillation frequency or to terminate the oscillations. The oscillation frequency, moreover, is determined spatially i.e. either by device geometry or illumination pattern or both.
Accordingly, it is an object of the present invention to provide an improvement in microwave and millimeter wave generators.
It is another object of the invention to provide an improvement in oscillators utilizing Gunn effect devices.
Yet another object of the invention is to provide an improvement in bulk semiconductor device Gunn oscillators.
Still another object of the invention is to provide a Gunn type oscillator providing a significant increase in RF peak power output level.